1. Field of the Invention
This invention relates to a rotational angle detector provided with a giant magnetoresistive element and a brushless or stepping motor using the detector.
2. Description of the Prior Art
The growth of the semiconductor technology and the digital technology in recent years has been encouraging all the industrial devices to succumb to digitalization and has been promoting the adoption of microcomputers therefor. Since motors which are capable of producing a digital positioning action are suitable as driving sources for these devices, stepping motors and servo motors have been heretofore used therefor. Likewise in the field of such applications as factory automation (FA) and robot, stepping motors which are smaller in size and larger in torque are used.
A conventional brushless or stepping motor (permanent magnet (PM) type stepping motor) 10, as illustrated in FIG. 1 and FIG. 2, is composed of a magnetic rotor (hereinafter referred to as "rotor") 12 formed of a permanent magnet and accommodated rotatably in a housing 11, exciting oils 14 disposed around the rotor 12 as opposed thereto across a prescribed gap, and rotational angle detecting magnetic sensors interposed between the exciting coils 14. By orderly exciting the exciting coils 14 externally, the rotor 12 is caused to change the position thereof depending on the state of coil electrization and produce a stepping motion, with the result that the angle of rotation and the speed of rotation will be controlled with the signal of excitation. It is not deniable, however, that the rotor 12 will possibly fail to rotate in spite of the excitation of the coils 14 on account of an unduly large load on the rotor 12. For exact control of the rotational angle of the rotor 12, therefore, a measure to use the magnetic sensors (Hall sensors) 15 for detecting changes in magnetic field of magnetized teeth 13 of the rotor 12 made of permanent magnet as the rotational angles or a measure to detect the rotational angles by the use of an encoder disposed outside the motor is resorted to.
In the miniaturization of the brushless or stepping motor, the permanent magnet rotor 12 provided with a plurality of magnetized teeth 13 and the housing 11 encompassing the exciting coils 14 can be miniaturized. The intervals between the individual teeth 13 of the rotor 12 of permanent magnet constructed as illustrated in FIG. 2 can be decreased to the order of some tens of .mu.m and, therefore, pose no problem to the miniaturization of the brushless motor. In the case of an application which requires the brushless motor to control the number of revolutions and the angle of rotation exactly, however, the miniaturization of this motor has its own limit because the Hall sensors 15 serving to detect the rotational angle cannot be reduced in size. The Hall sensors 15 each require at least four sensor leads (two voltage terminals 16a, 16b and two current terminals 17a, 17b) as illustrated in FIG. 3 (the detection magnetic field perpendicular to the face of the paper in the bearings of FIG. 3) and, therefore, entail as an inherent problem the noise of induced electromotive force due to a change in the magnetic field generated by the lead wires. Further, since the Hall sensors are unserviceable at elevated temperatures exceeding 100.degree. C. and since they are exposed, when operated in the stepping motor which usually gains in temperature while passing electric current, to such elevated temperatures, they are required to take into consideration the necessity for using a proper cooling means.
To overcome the problem, therefore, the idea of utilizing a magnetoresistive sensor 18 for which two sensor leads (current terminals 19a, 19b) suffice as illustrated in FIG. 4 (the detection magnetic field in an arbitrary direction) may be conceived.
The term "magnetoresistance (MR) effect" as used herein means a phenomenon that the electric resistance offered by a given material is varied by applying a magnetic field to that material. Generally, a ferromagnetic material is used as an MR element. A CoFe alloy having a rate of change of about 5% and a permalloy having a rate of change of about 2%, in magnetoresistance, are typical examples of the MR element. The rate of change of the magnetoresistance effect (magnetoresistance ratio, MR ratio) is expressed by the following formula (1): EQU Magnetoresistance ratio(%)=[R(O)-R(H)]/R(O).times.100 (1)
wherein R(O) represents the electric resistance in the absence of a magnetic field and R(H) represents the electric resistance in the presence of application of a magnetic field.
The utilization of the magnetoresistance effect is effective in realizing miniaturization of an angle detecting device as by reducing the number of necessary sensor leads to two and simplifying the layout of wires, for example. Examples of the motor which uses a rotational angle sensor utilizing the magnetoresistance effect are disclosed in published Japanese Patent Applications, KOKAI (Early Publication) No. 62-2353 and No. 8-289518. Since the brushless or stepping motor uses a magnet (having a surface magnetic field of not less than 100 [Oe]) as a rotor thereof and the exciting coil thereof for driving the rotor has a strong magnetic field (some hundreds of Oe), however, the sensor which utilizes a magnetoresistive element formed of a soft magnetic material represented by permalloy has the problem that it cannot detect the rotational angle because its detectable magnetic field (not more than some tens of Oe) is surpassed.